The invention relates to a method and to a device that is especially suitable for carrying out the method, for attenuation of load oscillations in a load mechanism with a controlled drive, in which a load is coupled mechanically to a motor via a spring element.
In drive control a load 2 is frequently moved via a shaft 4 by a controlled drive, consisting of a final control element, for example a power converter, and a motor 6. Such a drive configuration is shown in greater detail in FIG. 1. A further drive configuration is illustrated schematically in FIG. 2. In this further drive configuration the load 2 is moved via a transmission 8 by the motor 6 of a controlled drive. In this case it makes no difference whether a linear or a rotational movement of the motor 6 or the load 2 are involved and whether the transmission 8 converts a rotational movement into a linear movement or vice versa, whether a rotational movement is converted into another rotational movement or a linear movement is converted into another linear movement.
FIG. 3 shows a block diagram of a control path for a dominant natural frequency in greater detail. In this block diagram a current controller is identified by the number 10, a first integrator by the number 12, a second integrator by the number 14, a third integrator by the number 16, a fourth integrator by the number 18 and a subtractor by the number 20 and an adder by the number 22. The current controller 10 is supplied with a motor torque required value Msoll. The subtractor 20 is arranged between the current controller 10 and the first integrator 12. At the output of this first integrator an angular velocity actual value ωMist or the motor speed actual value is present, which is supplied to the second integrator 14. On the output side of this second integrator an angular actual value ΦMist or motor position actual value is present, which is supplied to a first input of the adder 22. The inverting input of this adder 22 is connected to the output of the fourth integrator, which is linked on its input side to an output of the third integrator 16. A load speed actual value ωList is present at the output of the third integrator, while a load position actual value ΦList is present at the output of the fourth integrator 18. These two integrators 12 and 14 form a motor mechanism 24, while the two integrators 16 and 18 exemplify a load mechanism 26. The motor mechanism 24 and the load mechanism 26 are coupled to one another by means of a shaft 4 or a transmission 8. In terms of control technology this mechanical coupling is exemplified by a spring component 28, of which the proportionality factor corresponds to the elasticity of the shaft 4 or of the transmission 8. The load position actual value ΦList present at the output of the fourth integrator 18 is connected to the adder 22 as negative feedback. The spring torque MF of the spring component 28 is switched to the subtractor 20 at which the motor torque actual value Mist is present. Moreover the spring torque MF is switched by means of a further adder 29 to a load torque ML.
Under particular circumstances of motor and load inertia and the elasticity of the shaft 4 or of the transmission 8, low-frequency oscillations, which are also referred to as load oscillations, arise between motor 6 and load 2. These load oscillations are frequently very disruptive and are difficult to manage with control technology.
By way of illustration the remarks given below are restricted to rotational movements, however the observations also relate in the same way to linear movements or to a mixture of linear and rotational movements.
State controllers are frequently used, in systems that are capable of oscillation, to attenuate such load oscillations. These controllers are however frequently so complex that it is only possible for them to be used by academic closed-loop control specialists. Such state controllers therefore appear unsuitable for a broad product solution, above all in respect of simple commissioning. If such automatic commissioning is unsuccessful manual intervention is only then possible by specialists. One problem in commissioning a state controller is that a number of feedbacks have to be adjusted simultaneously, since the feedbacks influence each other. This problem also occurs in principle when values to be connected to the torque are created, since a torque controller partly regulates away the value connected again and thus influences it. This could be prevented if the motor speed controller is adjusted slowly, however it would then be necessary to put up with a slow and less rigid controller. A good fault behavior is thus hardly successful. In technical literature the ‘virtual sensor’ is often propagated, which with a controlled model of the path capable of oscillation is to create the missing measurement variable. These approaches are complex and less robust in relation to path changes.
Another known solution uses a differential speed and differential position feedback to the angular velocity required value of the motor 6. Moreover a higher-ranking motor speed controller also delivers an angular velocity required value, which is added to the differential speed and the differential position feedback. Thereby a similar complex structure is obtained to that of the classic state controller, which is difficult to adjust. Since the motor speed controller in part compensates for the feedback values, the adjustment of the motor speed controller generally heavily influences the effect of the connection.
One way out could be to control the load speed directly without a motor speed controller. But this too is problematic, since the control path here has three poles at the stability edge and does not have any zero point, which makes a stable control possible only in a narrow band. For information only it should be pointed out here that a further pole at the stability edge is added by an I portion of the controller. This makes commissioning difficult.
At present there are no measures for active attenuation of load oscillations without load measurement. If no measures for active attenuation of load oscillations are employed, then an excitation of oscillation must be avoided by movement management. The consequence of this is that movement processes last comparatively long and that only a low control rigidity can be achieved. Faults can then excite oscillations that are not actively attenuated. There are in fact required value filters, which must be applied for all variables of the predetermined track, in order to avoid the excitation of the oscillation by required value changes. An excitation by a load torque change/fault will not be attenuated by these however.
An active attenuation of weakly attenuated natural frequencies by micro actuators can for example be carried out by a method that is described in German patent 102 46 093 C1 and is also known by the name of APC (Advanced Position Control). In some cases however active attenuation is not possible or is only possible with great difficulty.
The publication DE 101 37 496 A1 discloses a method and a control structure for attenuation of low-frequency load oscillations for drives with motor and load. A division of the controller cascades and an attenuation of the oscillation are undertaken only in the load speed controller. In this case it is not the motor torque, but a motor speed required value of a rapidly controlled motor speed controller that is selected as the connection point for a load acceleration.
The publication DE 103 40 400 A1 further discloses a method for attenuation of low-frequency load oscillations in a controlled drive, in which a load is coupled mechanically to a motor. From a signal of a speed control circuit of the controlled drive a natural frequency signal with a natural frequency of at least one low-frequency load oscillation to be attenuated is established. In addition this natural frequency signal is amplified as a function of a predetermined degree of attenuation and this amplified signal is applied as feedback to a predetermined motor speed required value. Thus a method is obtained for attenuating low-frequency load oscillations, which is no longer reliant on signals of the load circuit and which is an integral component of a drive controller of a controlled drive.